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Universal Shunt Ammeter
The Universal shunt shown below uses different
series-parallel combinations of resistances R1, R2, and R3 with a 1-ma full-scale
meter movement. The meter has three current ranges, namely: 2 ma, 10 ma, and 100 ma. The
total resistant of R1 (40 W), R2 (9 W) and R3 (1 W) is 50W which equals the resistance of
the meter movement. In the 2-ma range R1, R2, and R3 are in parallel with meter movement
so that 1-ma of current will flow through the resistance and 1-ma will flow through the
meter movement. In the 10-ma range R1 is placed in series with meter movement and R1 and
R3 are left as a shunt. In the 100-ma range R1 and R2 is placed in series with and R3 is
the shunt. In all cases 1-ma of current flows through the meter movement at full scale and
the remainder of the current flows through the shunt resistors.
The variable 0 through 28-Vdc power supply is used to provide a 1 to 100 ma current source with which to calibrate the Universal shunt meter. The external meter has a shunt resistance low enough (one order magnitude or more) to be considered zero and is calibrated to an accuracy of .3% of reading.
When testing 10 ma range the Universal Shunt Meter initially pegged off scale while external test ammeter read 10-ma. Adjusted power supply so that Universal meter read full scale, 10-ma. At this point external Ammeter reads 9.0 ma. All readings are in volts. Troubleshoot failure.
Note: You can download Video(avi) version of this GIF animation using buttons below.
Universal Ammeter with Fault (meter.avi)